Current practice for the design and construction of dams is such that their surplus water discharge works are designed for large floods (e.g. 1000- or 10,000-year flood). Consequently, only a very small portion of the said discharge works' flood discharge capacity is used most of the time. Furthermore, discharge over the sill is sometimes controlled by gates in order, mainly, to increase the storage capacity of the reservoir or increase the flood detention capacity of the dam.
In such arrangements, the said gates must obviously span the whole width of the overspill sill although most of them can remain almost always closed between occurrences of extraordinary floods and only be opened every twenty or fifty years for example. Where the second discharge structure can discharge the more frequent floods (as where the dam is provided with gated or uncontrolled surface spillways, submerged sluices or bottom outlets, or an intake to a hydro electric power-station or any other discharge works), all the said gates may clearly remain closed more or less permanently.
Whatever type of gate may be used, refusal of the gates to open is a major cause of dam failure. Therefore, gates are less reliable and safe than uncontrolled overspills, and they have the added disadvantage of being costly.
Various more economical means of closing off an overspill sill exist or have been proposed such as sandbags and flashboards or other similar arrangements which require human action prior to the arrival of each river flood, thereby involving a major risk of unsuccessful operation.
Some large embankment dams are provided with a fuse dyke or breaching section topped out at a lower level than the rest of the structure and operating by erosion of its constituent materials caused by the reservoir filling to its maximum level during an extraordinary flood. The purpose of the breaching section is to prevent uncontrolled catastrophic overtopping of the whole dam by an extraordinary flood, by concentrating the effects of the flood on a special section designed to be washed away by erosion, thus providing an extra discharge capacity. When the breaching section has been washed away, major reconstruction work is needed before the dam can resume normal operation. Furthermore, the disappearance of a breaching section may lead to an excessively fast rise in discharge in the lower river reach.
The applicant has already filed applications for U.S. patents for water level raising elements U.S. Pat. Nos. 5,032,038 and 5,061,118 granted on Jul. 16, 1991 and Oct. 29, 1991 respectively and both entitled "Overflow Spillway for Dams and Similar Structures"). These water level raising elements have the advantage of closing off the sill at low cost. However, in so far as they are designed to discharge small and moderate floods, their height must be less than the maximum reservoir level.
The problem which the invention seeks to solve is the provision of a means of near-permanent closure of all or part of the length of an overspill sill at a much lower cost than that of gates and over a greater height than heretofore while at the same time ensuring totally reliable and safe discharge of extraordinary floods automatically and without any major modifications to the structure. The invention is thus an economical substitute for those gates designed to open only on the advent of the more infrequent floods.
To the applicant's knowledge, there would seem to be no means currently in existence of satisfactorily fulfilling the goals stated hereinabove, of simple operation and moderate cost.